Currently, there are four general approaches to displaying three-dimensional images: (1) methods requiring special eyewear; (2) methods using three-dimensional display surfaces; (3) methods using holography; and (4) methods using parallax barriers and lenses.
(1) Special Eyewear
Several methods of three-dimensional imaging require special eyewear. The eyewear presents different images to a viewer's right and left eyes. Such methods involving only two different images are generally called “stereoscopic.” Many eyewear-based methods have a common source for right and left eye images, but the two different images are differentiated by two different types of lenses on the eyewear. For example, the lenses may differ by color, polarization, or sequential shutters. Other eyewear-based methods involve two different sources for right and left eye images, such as two independent mini-screens close to the eyes.
Disadvantages of special eyewear methods include: the inconvenience of having to wear special eyewear; the lack of multiple perspectives, occlusion, and image shifting in response to viewer movement; and eye strain or damage from conflicting convergence vs. accommodation cues that stress the human visual system. Lack of multiple perspectives, occlusion, and response to image shifting in response to viewer movement can be partially addressed by adding systems that track viewer motion, but these are also inconvenient and are difficult to apply in multi-viewer situations.
(2) Three-Dimensional Display Surfaces
Several methods of three-dimensional imaging use display surfaces that are themselves three-dimensional. Variations in these display surfaces include whether these surfaces are “full-scale” (on the same scale as displayed images) or “micro-scale” (on the same scale as pixels comprising the displayed images), whether these surfaces are stationary or moving, and whether these surfaces emit or reflect light.
Full-scale three-dimensional display surfaces are generally called “volumetric.” Stationary volumetric displays often include a series of parallel two-dimensional panels whose transparency can be varied. These panels emit or reflect light to create two-dimensional image slices which, when viewed together, form a three-dimensional image. Less commonly, a translucent gel can be used. Moving volumetric displays often have a spinning (or otherwise cyclically moving) two-dimensional structure that emits or reflects light. The light paths formed as its light emitting or reflecting members sweep through space create a three-dimensional image. Disadvantages of full-scale three-dimensional display surfaces include: they are cumbersome to construct and use for large-scale images with multiple viewers; and displays with transparent or translucent members produce transparent ghost-like images that are not desirable for many purposes.
Micro-scale three-dimensional display surfaces are less well-developed than full-scale surfaces and do not yet have a commonly-used label, but can be thought of as “three-dimensional pixels.” In theory, the concept of a three-dimensional pixel is a pixel comprised of an array of sub-pixels, each with image directionality as well as image content. Three-dimensional pixels could be in the form of a cube, sphere, or other shape. The concept of three-dimensional pixels has potential, but entails significant technical and practical challenges that have not yet been resolved. It is very challenging to create an extremely small structure with a sufficient number of fixed radiating “sub-pixels” to produce an image with reasonable resolution from different perspectives. Also, even if such structures of multiple “sub-pixels” can be created, it is very challenging to get them sufficiently close together for image precision without one structure blocking views of an adjacent structure. If the reader will pardon a colorful analogy, it is like trying to design a city block full of several-story apartment buildings wherein people in each apartment all want a view of the river; it is tough to do.
(3) Holographic Animation
Holographic animation has tremendous potential, but is still at an early stage with many technical challenges yet unresolved. Current systems for animated holographic imaging produce relatively small translucent images with limited viewing zones and poor image resolution. They also require coherent light with associated expense and safety concerns. Some day holographic animation may become the method of choice for three-dimensional imaging, but thus far it remains very limited.
(4) Parallax Barriers and Lenses
There are many methods of three-dimensional imaging using parallax barriers, lenticular lenses, fly's eye lenses, variable focal-length lenses, and combinations thereof.
Parallax barriers allow different images to reach a viewer's right and left eyes by selectively blocking portions of images, generally via a layer that is close to the image surface. Light-blocking vertical strips and light-transmitting vertical slits are often used as parallax barriers. Some parallax barriers are stationary. Other parallax barriers move in response to viewer head motion in systems that track this motion. Lenticular lenses are (semi-circular) columnar lenses. They are generally combined in vertical arrays near an image surface. Lenticular lenses direct different views (generally vertical image strips) to a viewer's right and left eyes. Parallax barriers and lenticular lenses can be used together.
Lenticular lenses and parallax slits only provide parallax in one direction. Some parallax in another direction can be achieved by adding a viewer head tracking system and varying image content to reflect viewer head motion, but this is cumbersome for one viewer and problematic for multi-viewer applications. Another disadvantage of parallax barriers and lenticular systems are “pseudoscopic” images outside a severely-restricted viewing zone. “Pseudoscopic” views occur when the images that the eyes see are improperly reversed. “Pseudoscopic” views can cause eye strain, headaches, and other health problems.
A “fly's eye” lens is an array of convex lenses. Three-dimensional imaging using a fly's eye lens is called “integral photography.” A fly's eye lens can display a large number of small two-dimensional images from different perspectives. Ideally, as a viewer moves, the viewer sees the same point from different perspectives. Although this concept has considerable potential, it involves significant practical challenges. It is difficult to have a sufficient number of two-dimensional images to achieve high image resolution on a very small scale structure. Viewing zones remain limited. Production of fly's eye screens is also relatively expensive.
New methods have also been proposed for creating three-dimensional images using lenses whose focal lengths can be changed in real time. Such lenses include electro-wetting controlled droplet lenses and liquid-crystal microlenses. Lenses whose focal lengths can be changed are called “dynamic” or “active” lenses. Although application of such lenses to the creation of three-dimensional images has considerable potential, there remain many technical challenges. Systems to independently adjust the focal lengths of a large number of microlenses are complex. Liquids may not move sufficiently rapidly to adjust focal length fast enough for three-dimensional viewing. Viewing zones remain limited.
(5) Summary of Background and Related Art
To summarize the related art, considerable work has been devoted to create ways to display three-dimensional images. However, all of the current methods still have disadvantages. Some methods require inconvenient eyewear and cause eye strain. Some methods require viewer tracking that is inconvenient and does not work well for multiple viewers. Some methods have restrictive viewing zones. Some methods produce transparent, ghost-like images. Some methods produce very small, low-resolution images and require use of coherent light. Some methods have significant unresolved technical challenges concerning the creation of complex microstructures. Some methods do not adjust rapidly enough to display moving three-dimensional images. None of the current methods provide a practical means to create high-resolution, large-scale, moving, three-dimensional images that can be viewed by people in different locations, with full parallax, without special eyewear. The invention disclosed herein addresses these disadvantages.